In this application, it is proposed to study the role of a mammalian subtilisin-like protease, furin, in processing and activation of retroviral glycoprotein precursors, as well as in the normal development of the mouse. The furin-encoding gene (fur) will be disrupted by homologous recombination, in both somatic cells and pluripotent embryonic stem (ES) cells and these mutant cells will be used for both in vitro and in vivo experiments. Human T cells as well as human epithelial cells in which one or both of the fur gene have been disrupted will be used for retroviral glycoprotein gene expression and virus infectivity experiments. This approach will provide an experimental test of the hypothesis that furin is the enzyme responsible for cleavage/activation of the HIV and other retroviral glycoproteins. Because mouse ES cells allow the reconstitution o chimeric progeny, these approaches will also allow studies on the role of the ubiquitously expressed furin in the process of mouse development and normal homeostasis. In combination the information gained should provide new avenues for the development of anti-retroviral therapeutics.